fix: rollback aborted PG transaction on user delete FK violation

PostgreSQL enters 'current transaction is aborted' state after a
ForeignKeyViolation. Without rollback(), every subsequent query on the
shared connection fails with 'commands ignored until end of transaction
block'. Now both db.py and server.py rollback on the error path.

app/src/ctxd/db.py

@@ -194,6 +194,7 @@ def user_delete(conn, user_id: str) -> dict:
         if _is_pg(conn):
             import psycopg
             if isinstance(e, psycopg.errors.ForeignKeyViolation):
+                conn.rollback()  # clear aborted transaction state
                 return {"ok": False, "error": "user_has_references", "hint": "Inactivate the user instead of deleting."}
             raise
         return {"ok": False, "error": "user_has_references", "hint": "Inactivate the user instead of deleting."}

app/src/ctxd/server.py

@@ -1105,6 +1105,7 @@ class HTTPServer:
                 return (400, {"Content-Type": "application/json"}, json.dumps({"error": "cannot delete your own account"}))
             result = _db.user_delete(self._conn, uid)
             if not result.get("ok"):
+                self._conn.rollback()
                 status = 404 if result.get("error") == "not_found" else 409
                 return (status, {"Content-Type": "application/json"}, json.dumps(result))
             actor = self._auth_user_id(auth)